fix: remove unused storage maps NextStakeJobId, UsedWork, LargestLocked

[Nicknamess96]

Summary

Removes three storage maps that are either completely unused or write-only, reducing chain state bloat:

• NextStakeJobId — declared but never read or written anywhere; its companion StakeJobs map does not exist
• UsedWork — written during PoW registration (do_burned_registration, do_registration) but never read, so the intended work-replay protection was never enforced
• LargestLocked — written during genesis and removed during network dissolution, but never read in production code

Audit methodology

Full ripgrep search across the repository for each identifier (type name, snake_case variants) confirmed:

• No reads (.get(), .try_get(), .contains_key(), ::iter()) in production code
• No references in runtime APIs, RPC implementations, or metadata
• LargestLocked has test-only reads in tests/networks.rs (removed alongside the map)

Files changed

File	Change
pallets/subtensor/src/lib.rs	Remove 3 storage declarations
pallets/subtensor/src/subnets/registration.rs	Remove 2 dead UsedWork::insert calls
pallets/subtensor/src/macros/genesis.rs	Remove LargestLocked::insert in genesis
pallets/subtensor/src/coinbase/root.rs	Remove LargestLocked::remove in dissolution
pallets/subtensor/src/tests/networks.rs	Remove test setup/assertion for LargestLocked

Closes #2397

Test plan

• Existing tests pass (no production code reads these maps)
• cargo clippy clean
• Verify no downstream RPC consumers depend on these storage keys